This invention relates to an electronic time base measurement circuit and in one of its aspects to an electrical circuit for performing such measurements in an ultrasonic inspection system.
Application of a voltage pulse to a piezoelectric crystal will cause it to mechanically oscillate at its resonant frequency. Sound, at the reasonant frequency of the crystal will, when it strikes the crystal, also cause oscillation. When the crystal oscillates, it generates a sinusoidal voltage and a high frequency sound wave, both of which occur at the crystal's resonant frequency.
In ultrasonic flaw-detector and thickness measuring instruments, a piezoelectric crystal (transducer) is placed on or near the surface of the material whose integrity or thickness is to be measured. In order to insure effective coupling between the transducer and the surface, the space between may be filled with an acoustically transparent material, i.e., a material having a small amount of accoustical attenuation such as water or oil. The output of a pulse generator, consisting of short duration voltage pulses, is applied to the crystal. In thickness measuring instruments, high frequency sound generated by the crystal when it is pulsed passes through the material, is reflected from the opposite surface, and returns to the crystal where the back-reflected sound causes the crystal to again oscillate. The same sequence of events may happen repeatedly and there may be a second, third or even greater number of back-reflections due to the same voltage pulse. The voltage pulse initiating this sequence is called the initial pulse. By measuring the elapsed time between the initial pulse and a back-reflection or between two back-reflections, and knowing the velocity of sound through the material being tested, the thickness can be determined.
In ultrasonic pulse-echo applications it is often desirable to accurately measure the time required for the transmitted pulse to traverse the material under test, be reflected by the back surface or a defect within the material, and return to the receiving transducer. This time measurement may be used to calculate the thickness of the material or the location of a defect. Due to the high velocity of sound in most materials, this time measurement must be made very precisely if thickness measurements are to be performed with acceptable accuracy. For example, a timing error of one millionth of a second can result in a calculated thickness in error of about one-sixteenth inch in steel.
Ultrasonic pulse-echo thickness measurement apparatus generally consists of a highly damped piezoelectric transducer excited by an ultrasonic pulse generator connected thereto, injects ultrasonic pulses of short duration into a specimen such as a plate metal, to determine the thickness D thereof. After entering the specimen, the ultrasonic pulse is repeatedly reflected back and forth between the parallel surfaces of the specimen separated by the dimension D until its energy is dissipated. During this reverberation process, piezoelectric transducer which also acts as an ultrasonic receiver, generates a short voltage pulse each time the ultrasonic pulse strikes upon the specimen surface to which the piezoelectric transducer is coupled. Thus, following the emission of the initial excitation pulse, a sequence of electrical pulses is produced by the piezoelectric transducer. The time interval T between two consecutive pulses of this sequence is equivalent to the specimen thickness according to EQU T= 2D/V.sub.L
where V.sub.L represents the longitudinal ultrasonic wave velocity in the material of the specimen. For a specific material, the longitudinal ultrasonic wave velocity is usually constant within a wide range of ultrasonic frequencies and the specimen thickness D can be determined by measuring the pulse period T or its reciprocal.
The time interval between the initial pulse and a back-reflection or between various reflection pulses can be determined by displaying on an oscilloscope the sinusoidal voltage across the crystal corresponding to the initial pulse and back-reflections. Thickness of the material being tested was then read on the horizontal or time axis of the oscilloscope. A more recent development is the direct-reading instrument which displays thickness measurements directly on a meter or on a digital read-out display. In the direct-reading instrument, a constant current source is used to charge a capacitor at a linear rate with respect to time. The constant current source is gated-on by the initial pulse and gated-off by the first back-reflection. The charge on the capacitor is, therefore, dependent on the elapsed time between the initial pulse and the first back-reflection which, in turn, depends on the thickness of the material. The charge on the capacitor at any time is indicated on a meter or a digital-type display. The readout, whether meter or digital-type, is calibrated directly in inches. Both the oscilloscope and direct-reading type instruments require considerable electronic circuitry and, as a consequence, have a large physical size. Both of these instruments are also relatively expensive.
This invention provides a simple and novel method and apparatus for measuring the elapsed time between two consecutive signals and is not limited to ultrasonic testing. In one specific embodiment, it provides a method and apparatus for measuring the elapsed time between an initial pulse and the first back-reflection or between various reflection pulses in ultrasonic testing. The apparatus is less expensive and much smaller than the instruments currently in use. This invention provides an accurate ultrasonic thickness measuring device which is made from simple, inexpensive electronic components and which can be easily hand held.